Open Stack

On 11/13/2013 11:40 AM, Jiang, Yunhong wrote:

>> But, from performance point of view it is better to exclusively
>> dedicate PCPUs for VCPUs and emulator. In some cases you may want
>> to guarantee that only one instance(and its VCPUs) is using certain
>> PCPUs.  By using core pinning you can optimize instance performance
>> based on e.g. cache sharing, NUMA topology, interrupt handling, pci
>> pass through(SR-IOV) in multi socket hosts etc.
>
> My 2 cents. When you talking about " performance point of view", are
> you talking about guest performance, or overall performance? Pin PCPU
> is sure to benefit guest performance, but possibly not for overall
> performance, especially if the vCPU is not consume 100% of the CPU
> resources.

It can actually be both.  If a guest has several virtual cores that both 
access the same memory, it can be highly beneficial all around if all 
the memory/cpus for that guest come from a single NUMA node on the host. 
  That way you reduce the cross-NUMA-node memory traffic, increasing 
overall efficiency.  Alternately, if a guest has several cores that use 
lots of memory bandwidth but don't access the same data, you might want 
to ensure that the cores are on different NUMA nodes to equalize 
utilization of the different NUMA nodes.

Similarly, once you start talking about doing SR-IOV networking I/O 
passthrough into a guest (for SDN/NFV stuff) for optimum efficiency it 
is beneficial to be able to steer interrupts on the physical host to the 
specific cpus on which the guest will be running.  This implies some 
form of pinning.

> I think pin CPU is common to data center virtualization, but not sure
> if it's in scope of cloud, which provide computing power, not
> hardware resources.
>
> And I think part of your purpose can be achieved through
> https://wiki.openstack.org/wiki/CPUEntitlement and
> https://wiki.openstack.org/wiki/InstanceResourceQuota . Especially I
> hope a well implemented hypervisor will avoid needless vcpu migration
> if the vcpu is very busy and required most of the pCPU's computing
> capability (I knew Xen used to have some issue in the scheduler to
> cause frequent vCPU migration long before).

I'm not sure the above stuff can be done with those.  It's not just 
about quantity of resources, but also about which specific resources 
will be used so that other things can be done based on that knowledge.

Chris